Re: vmx real mode emulated mmio?

[Gleb Natapov <gleb@redhat.com>]

On Fri, Dec 14, 2012 at 04:42:36PM -0700, Alex Williamson wrote:
> 
> I don't really know what I'm doing messing around with realmode
> exception handling, but are we missing something like this:
> 
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -4332,7 +4332,8 @@ static int handle_exception(struct kvm_vcpu *vcpu)
>                 if (vcpu->arch.halt_request) {
>                         vcpu->arch.halt_request = 0;
>                         return kvm_emulate_halt(vcpu);
> -               }
> +               } else if (vcpu->mmio_needed)
> +                       return 0;
>                 return 1;
>         }
>  
> 
> The background is that I was playing around with when the MMIO regions
> of PCI assigned devices get direct mapped through KVM and start using
> memory slots and I found that the boot ROM on an e1000e card that I
> often use for testing causes the VM to hang when not directly mapped
> through a memory slot.  This happens with both pci-assign and vfio-pci.
> Debugging further, I see the kernel instruction emulator decodes this
> and sets all the necessary flags and fragments for userspace to complete
> the access, but since it happened in real mode, vcpu_enter_guest always
> returns 1 and therefore KVM_RUN never exits to userspace and KVM gets
> stuck in an infinite loop re-handling this instruction.
> 
> The above is pretty trivial and allows the ROM to work again.  Is it
> correct?  What other exceptions are we missing here that should allow an
> exit?  This particular access generated a GP_VECTOR exception if we want
> to key on that somewhere.  Thanks,
> 
As far as I see the patch is nop. If handle_rmode_exception() returns 1
mmio_needed should be 0. Also if handle_rmode_exception() makes IO
according to the code qemu should exit with "KVM: exception 13 exit
(error code 0x0)" or something. Can you provide a ftrace for the hang?

--
			Gleb.